1. Field of the Invention
This invention relates to a gray level expression method for a plasma display panel, and more particularly to a method and apparatus for expressing a gray level with a decimal value in a plasma display panel that is capable of enhancing a picture quality.
2. Description of the Related Art
Generally, a plasma display panel (PDP) radiates light from phosphors excited by an ultraviolet ray generated during a gas discharge, thereby displaying a picture including characters and graphics. Such a PDP is easy to be made into a thin-film and large-dimension type. Moreover, the PDP provides a very improved picture quality owing to a recent technical development.
Referring to FIG. 1, a conventional three-electrode, AC surface-discharge PDP, which is hereinafter referred to as “three-electrode PDP”, includes a scanning electrode Y and a sustaining electrode Z provided on an upper substrate 10, and a data electrode X provided on a lower substrate 18.
The scanning electrode Y and the sustaining electrode Z. have transparent electrodes 12Y and 12Z with a large width and metal bus electrodes 13Y and 13Z with a small width, respectively, and are formed on the upper substrate in parallel. An upper dielectric layer 14 and a protective film 16 are disposed on the upper substrate 10 in such a manner to cover the scanning electrode Y and the sustaining electrode Z. Wall charges generated upon plasma discharge are accumulated in the upper dielectric layer 14. The protective film 16 prevents a damage of the upper dielectric layer 14 caused by a sputtering during the plasma discharge and improves the emission efficiency of secondary electrons. This protective film 16 is usually made from magnesium oxide (MgO). The data electrode X is crossed to the scanning electrode Y and the sustaining electrode Z.
A lower dielectric layer 22 and barrier ribs 24 are formed on the lower substrate 18. The surfaces of the lower dielectric layer 22 and the barrier ribs 24 are coated with a fluorescent material layer 26. The barrier ribs 24 separate discharge spaces being adjacent to each other in the horizontal direction to thereby prevent optical and electrical crosstalk between adjacent discharge cells. The fluorescent layer 26 is excited by an ultraviolet ray generated during the plasma discharge to generate any one of red, green and blue visible light rays. An inactive mixture gas of He+Xe, Ne+Xe or He+Xe+Ne is injected into a discharge space defined between the upper and lower substrate 10 and 18 and the barrier rib 24.
In a PDP, one frame is divided into a plurality of sub-fields which are different from each other in the number of discharge, so as to realize gray levels of a picture. Each sub-field is again divided into a reset period for uniformly causing a discharge, an address period for selecting the discharge cell and a sustaining period for realizing the gray levels depending on the discharge frequency.
For instance, when it is intended to display a picture of 256 gray levels, a frame equal to 1/60 second (i.e. 16.67 msec) is divided into 8 sub-fields SF1 to SF8 as shown in FIG. 2. Each of the 8 sub-fields SF1 to SF8 is again divided into a reset period, an address period and a sustaining period. The reset period and the address period of each sub-field are equal every sub-field. The address discharge for selecting the cell is caused by a voltage difference between the data electrode X and the scanning electrode Y. The sustaining period is increased at a ration of 2n (wherein n=0, 1, 2, 3, 4, 5, 6 and 7) at each sub-field. A sustaining discharge frequency in the sustaining period is controlled at each sub-field in this manner, to thereby realize gray levels.
FIG. 3 illustrates driving waveforms applied to the scanning electrode Y, the sustaining electrode Z and the data electrode X at the first to third sub-fields having a low brightness weighting value.
Referring to FIG. 3, a reset period for initializing a panel is assigned at an initial time of the frame. In the reset period, a high positive reset pulse RST is applied to the sustaining electrode Z to cause a reset discharge within cells of the panel. Since this reset discharge allows wall charges to be uniformly accumulated in the cells of the panel, a discharge characteristic becomes uniform.
Each of the first to third sub-fields SF1 to SF3 includes an address period, a sustaining period and an erase period. Herein, the address periods and the erase periods are set equally, whereas the sustaining periods become different depending upon a brightness weighting value given to each sub-field SF1 to SF3.
The first sub-field SF1 has a brightness weighting value set to 20. In the address period of the first sub-field SF1, a data pulse DATA is applied to the address electrode X and a scanning pulse −SCN is sequentially applied to the scanning electrode Y in such a manner to be synchronized with the data pulse DATA. A voltage difference between the data pulse DATA and the scanning pulse −SCN is added to a wall voltage within the cells, thereby allowing the cells supplied with the data pulse DATA to cause an address discharge. In the sustaining period of the first sub-field SF1, a sustaining pulse is once applied to each of the scanning electrode Y and the sustaining electrode Z in correspondence with the brightness weighting value ‘20’. The cells selected in the address period are discharged for each sustaining pulse while the sustaining pulse being added to an internal wall voltage to thereby have total twice discharge. In the erase period of the first sub-field SF1, an erase signal ERASE with a shape of ramp wave is applied to all the scanning electrodes Y. This erase signal ERASE erases a sustaining discharge and uniformly forms a certain amount of wall charges within the cells of the panel.
The second sub-field SF2 has a brightness weighting value set to 21 while the third sub-field SF3 has a brightness weighting value set to 22. The address periods of the second and third sub-fields SF2 and SF3 cause an address discharge within the cells supplied with the data pulse DATA in similarity to that of the first sub-field SF1 to select the cell. In the sustaining period of the second sub-field SF2, a sustaining pulse is twice applied to each of the scanning electrode Y and the sustaining electrode Z in correspondence with the brightness weighting value ‘21’. In the sustaining period of the third sub-field SF3, a sustaining pulse is four times applied to each of the scanning electrode Y and the sustaining electrode Z in correspondence with the brightness weighting value ‘22’. Accordingly, total four times discharge are generated at each of the cells selected by an address discharge in the sustaining period of the second sub-field SF2, whereas total eight times discharge are generated at each of the cells selected by an address discharge in the sustaining period of the third sub-field SF3.
The conventional PDP driving method has a problem in that it is unable to express a gray level less than 1. More specifically, the conventional PDP expresses a gray level with an integer value by a combination of sub-fields, to each of which a brightness weighting value of an integer is set, as seen from the following Table 1. A brightness weighting value of each sub-field becomes equal to the number of sustaining pulse pairs.
The following Table represents on/off of the sub-field according to a gray level value in the case of 8-bit default code.
TABLE 1SF1 (1)SF2 (2)SF3 (4)SF4 (8)SF5 (16)SF6 (32)SF7 (64)SF8 (128)0xxxxxxxx10xxxxxxx2x0xxxxxx300xxxxxx4xx0xxxxx...........................126x000000x1270000000x128xxxxxxx0...........................252xx0000002530x000000254x000000025500000000
In the Table 1, the uppermost row represents sub-fields, and their brightness weighting values and the leftmost column represents the number of sub-field pairs. Further, ‘0’ means turned-on sub-fields SF1 to SF8 while ‘x’ means turned-off sub-fields.
As can be seen from the Table 1, the conventional PDP cannot express a gray level with a value of less than 1. Particularly, if an input image signal undergoes an inverse gamma correction, then it becomes impossible for the PDP to express a part of low gray levels in the input image signal because low gray levels, for example, gray levels smaller than ‘21’ are changed into gray level values less than ‘1’ as shown in FIG. 4. Also, if an input image signal undergoes an error diffusion after the inverse gamma correction, then a data converted into a gray level value less than ‘1’ by the inverse gamma correction is displayed by so-called “error diffusion artifact” acting as a point pattern noise due to an error diffusion component diffused into the adjacent cells. As a result, if an input image having a dark object moved within a field having a dark background is displayed on the PDP, then it becomes impossible to exactly identify a shape of the dark object because the moving dark object is displayed by error diffusion artifact.
Recently, there has been developed a driving system of controlling the total number of sustaining pulses depending upon an average brightness of an input image. As seen from the following Table 2, this average image control system reduces the total number of sustaining pulses with respect to any one of sub-field arrangements with a different number of total sustaining pulses when an average brightness of an input image is high, whereas it enlarges the total number of sustaining pulses when an average brightness of an input image is low. Likewise, in this case, if a field having a high average brightness undergoes an inverse gamma correction and an error diffusion, then it becomes impossible to express a decimal value of gray levels, particularly, gray levels less than 1.
TABLE 2SF1SF2SF3SF4SF5SF6SF7SF8SF9SF1010231248163264128256512 511—124 81632 64128256 255——12 4 816 32 64128
In the Table 2, the uppermost row represents sub-fields, and the leftmost column represents the total number of sustaining pulse pairs. As can be seen from the Table 2, if the number of sustaining pulse pairs is 255, then it becomes impossible to express a decimal value of gray levels.